This project will determine the effect on mutation frequency of repair of DNA following alkylation of DNA in germ cells of Drosophila melanogaster. Previous work in this laboratory has shown a rapid loss of alkyl groups from DNA coupled with a low probability of mutation in early germ cell stages and a slow rate of loss of alkyl groups coupled with a high mutation frequency in later germ cell stages. This correlation between mutation induction and the apparent activation of different repair systems in different germ cell stages will be tested by substitution of known repair-deficient mutants into strains of D. melanogaster for which the rate of loss of alkylation and mutational response to alkylation are known. This test will show 1) whether mutant alleles which have been shown to be repair-deficient in tissue culture are also repair-deficient in vivo in germ cells, and 2) whether the repair deficiency enhances mutation induction in specific germ cell stages. By using two mutagens -- ethyl methane sulfonate (EMS) and N-ethyl-N-nitrosourea (ENU) -- which differ greatly in their ratios of N7 to O6 alkylation of guanine we will also assess the relative importance of these two sites of alkylation in the mutation process.